The invention concerns a method for the manufacture of shaped bodies by extrusion moulding of plastic-filler mixtures containing more than 50% by volume of fillers and to an extruder for carrying out the method.
The property profile of plastics can be changed and as a rule improved by the addition of fillers. Improvements of this type concern, for example, the processability of the plastics or the properties of the products manufactured from such plastic-filler compositions such as E-modulus, impact strength, dimentional stability or heat resistance. As long as the content of fillers does not exceed a stipulated amount, no problems occur in the shaping of the plastic-filler materials to intermediate or end products. Above a specific content of fillers, the flow qualities of such materials however degrade increasingly and their processability becomes increasingly more difficult. This has especially problematical effects on extrusion moulding. With high filler contents, the limiting transverse stresses necessary for generating a shearing flow rapidly become disproportionally high and very high forces are necessary for the extruding. Extrusion of this type can only be handled with a correspondingly expensive machine tool equipment, that is with special machines, and in many cases such materials generally cannot be extrusion moulded. Plastic materials with high filler contents have, moreover, a low melt dilatability. On emergence from the nozzle moulding tool, damage like tearing or roughening or scaly surfaces therefore occurs, in particular with the extrusion moulding of flat bodies or even of tubes.